1. Field of the Invention
The present invention relates to an optical fiber coupler used in the field of optical communications, a method for manufacturing the optical fiber coupler and an optical amplifier using the same.
2. Prior Art
In optical communication systems, optical fiber amplifiers have been used for directly amplifying light signals.
In a conventional optical fiber amplifier, for example, as shown in FIG. 7, a rare earth doped optical fiber 1b from a pumping light source 6 through a splicing portion 9 and another optical fiber 1a for input of signal light have been connected by fusing to a side of the multiplexer (WDM) 7. The rare earth-doped fiber 4 for output of the amplified signal has been connected on the other side of the multiplexer. These doped fibers are doped with a rare earth element such as Er or Nd in the optical material of the core fiber.
In general, for the amplifier, a passive component such as inline-type optical isolator 8 has been connected to the doped fiber 4 via another splicing portion 9 by fusing on the output side of the said multiplexer (WDM) 7, as shown in FIG. 7.
The multiplexer 7 comprises a transmission optical fiber which transmits light from the pumping light source 6 having wavelength shorter than the signal light. One end of the transmission fiber is connected through splicing portion 91 with the rare earth-doped fiber 4 which on the out put side is connected in a splicing portion 92 with a single-mode fiber 1 of the inline type optical isolator 8.
In such an optical amplifier, the multiplexer 7 multiplexes the signal light and the pumping light wherein both light rays enter the rare earth-doped fiber 4 so that the pumping light pumps rare earth ions in the rare earth-doped fiber 4, thereby generating the state of reversed distribution of energy level and amplifying the signal light. The amplified signal light is orientated by the inline type optical isolator 8 to the output side.
However, in the optical amplifier described above, every fiber type device constructing the amplifier is connected by fusing in the splicing portions 9, 91 and 92, and therefore, fabrication of such an amplified has required much labor for splicing fibers by fusing. Connection of fibers each other also leads to a loss in optical power, then resulting in decreased gain and deteriorated signal to noise ratio, which are to be detrimental for optical amplification.
In the construction of the prior art amplifier above, since the signal light and the pumping light enter the rare earth-doped fiber 4 after being multiplexed in the multiplexer 7, it has been difficult to connect the fibers while setting the different mode field diameters of the optical fibers. Also it has been necessary to secure a space for accommodating the splicing portions 9, 91 and 92 when installing the optical circuit as a amplifier module, thus causing a problem of increased space for installing the optical circuit.